Question

In negative operon
A. Co-repressors binds with repressor
B. Co-repressor does not bind with repressor
C. Co-repressor binds with inducer
D. cAMP has negative effect on lac operon

Answer 1

Operon is a functional unit of DNA, containing a set of genes under the control of a single promoter. In negative operons, the operon is turned off in the presence of a repressor.

Complete answer:
Option A: An operon segment contains a regulatory sequence, a promoter sequence and the set of genes that are under the control of this promoter. Promoter is the binding site of RNA polymerase which will transcribe the downstream genes.
Binding of an inducer protein to the repressor changes conformation of the repressor which will lead to the release of repressor from the operator. Which will lead to the binding and transcription of genes by RNA polymerase.
Binding of repressor protein to the operon prevents the operon from functioning.Therefore the transcription of genes doesn't occur. Co-repressor binding with repressors will stabilise the repressor binding and prevent the repressor from falling off from the operon. This prevents any basal level transcription.
In the case of negative regulation, co-repressor binds with the repressor. Therefore this is the correct option.

Option B: If the co-repressor does not bind with the repressor, there are chances of the repressor falling off from the operon and a leaky transcription will result from this. Therefore, this is the incorrect option.
Option C: Co-repressor binds with repressors.Therefore, this is the incorrect option.
Option D: CAP is a glucose sensor of lac operon which senses glucose indirectly through the help of cAMP molecules. When glucose levels are low, CAP activates the transcription of the operon. cAMP doesn't have a negative effect on operon. Therefore, this is the incorrect option.

Hence, the correct answer is option (A)

Note: 1.Operons are present in some viruses as well
2.Lac operon is an operon responsible for the transport and metabolism of lactose in E.coli